1. Field of the Invention
The present invention relates generally to a transfer of thermal energy, and more particularly, to heat transfer arrangements employing a membrane desiccation heat pump for both heating and cooling applications.
2. Description of the Prior Art
A cooling of a gaseous fluid containing humidity and, in particular, a cooling of humid air, is desired in many circumstances. Cooling of ambient atmosphere is often desired in buildings, domestic dwellings, in appliances such as refrigerators, and in storage rooms and the like. It is also desired in delivery vehicles and trucks, and in aircraft and marine craft. Other applications are natural gas cooling for removal of natural gas liquids or providing of controlled inert atmosphere to industrial processes such as paint drying, food drying and clean rooms.
Cooling systems using water vapor as working fluid are among the oldest in the art of producing cold. Early processes made use of principles of both compression and adsorption refrigeration, the latter using sulfuric acid as an absorbent. Vacuum refrigeration systems using water vapor or organic vapor as a refrigerant and steam injector as a compressor were well adapted to air conditioning application in the 1940s.
Many different proposals have been made for cooling gases and, in particular, for the cooling of air. A popular system in widespread use utilizes a compression and expansion of a heat exchange medium in the form of a gas, which can be compressed into a liquid state, and then is allowed to expand into a vapor state, i.e., the so-called compression/expansion cycle. In most cases, chloro-fluoro carbon gases, e.g., Freon™, were used, but recently such gases have been considered environmentally unsafe. Conventional alternatives to Freon™ are not as efficient as Freon™, and thus systems using the compression-expansion cycle require a relatively large input of power for the compression cycle. As a result, attention has turned to the feasibility of air conditioners that rely on alternative energy sources. Desiccant air conditioning systems are able to utilize alternative sources such as waste heat or solar energy for cooling and air conditioning thereby reducing electric power consumption and reducing reliance upon conventional power sources.
Desiccant air conditioners work as follows. During a cooling mode or adsorption cycle, hot humid air enters an intake side of an air conditioning system and passes through one side of a slowly turning desiccant wheel or circular desiccant bed. Water vapor and other moisture vapor are adsorbed on an extended desiccant material surface area, drying the air and releasing latent heat of condensation. Hot dry air from the desiccant bed wheel then passes through a heat exchanger such as an air-to-air heat exchanger wheel giving up some of the heat to an exhaust air stream. The air is then reconditioned to be in a desired comfort zone by passing through an evaporative element or unit where moisture is evaporated back into the air, for example by spraying, cooling the air to a desired temperature and humidifying the air to a desired relative humidity.
Open cycle desiccant systems have been known from the early 1940's. In 1955, U.S. Pat. No. 2,700,537 to Pennington described using rotary heat exchangers impregnated with desiccants. Today dual path machines still use the Pennington cycle. In 1960, U.S. Pat. No. 2,926,502 to Munters improved this cycle. The '502 patent discloses an air conditioning system including the recycling of air, at least three air flow paths, with all embodiments including a recycling of interior space conditioned air path, an open cycle regeneration path and a supplementary air path for an additional heat exchanger.
U.S. Pat. No. 4,594,860 to Coellner et al. discloses an open cycle desiccant air conditioning system in which the regeneration path is an open cycle and very similar to Pennington's cycle. U.S. Pat. No. 2,186,844 to Smith discloses a refrigeration apparatus wherein heat from a mechanical refrigeration unit regenerates desiccant, very similar to concepts described in U.S. Pat. No. 5,502,975 to Brickley et al. and U.S. Pat. No. 5,517,828 to Calton et al. The common factor is the open cycle regeneration path. U.S. Pat. No. 4,786,301 describes an air conditioning system having heat exchanging desiccant bed with alternating adsorption/desorption cycles, an improvement of this concept is described in U.S. Pat. No. 5,222,375 entailing the use of two alternating desiccant beds.
U.S. Pat. No. 5,353,606 to Yoho et al. addresses a three-path desiccant air conditioning system. As with other prior art systems, all these regeneration paths are open cycle.
Some of the problems associated with desiccant air conditioners are the need for removal of latent heat of condensation and adsorption from the desiccant bed and desiccant material during the adsorption cycle, the need for thermal energy for the desiccant regeneration cycle and the need to cool the desiccant after the regeneration. Furthermore, desiccant wheel machines are cumbersome to build and require a high level of maintenance.
Several authors have suggested employing a heat pump to raise the temperature of heated fluid for utilization of a waste heat stream. Heat pump systems employed for such processes are based on conventional cycles, i.e., fluid compression, absorption, sorption and desiccation cycles. The use of heat pumps for heat recovery have been shown in several applications utilizing various low level heat sources such as heat emitted by refrigerators (U.S. Pat. Nos. 4,041,724 and 4,226,089), an exhausted air duct (U.S. Pat. Nos. 4,100,763, 4,175,403 and 4,416,121), paper mill processes (U.S. Pat. Nos. 4,026,035, 4,437,316, 4,522,035 and 4,780,967), a power plant (U.S. Pat. No. 4,124,177), solar energy (U.S. Pat. Nos. 4,143,815, 4,332,139 and 4,703,629). Other sources of low level heat are: a plurality of secondary heat sources from an industrial plant or a factory (U.S. Pat. Nos. 4,173,125, 4,307,577, 4,333,515 and 5,548,958.); humid air (U.S. Pat. Nos. 4,197,713 and 4,517,810); air exhausted from a paint spray booth (U.S. Pat. No. 4,197,714); a gas stream of a drying oven (U.S. Pat. No. 4,295,282); a building stack or a flue (U.S. Pat. Nos. 4,314,601 and 4,660,511); waste heat from a gas turbine (U.S. Pat. No. 4,347,711); thermally activated separation processes such as fractional distillation, distillation, dehydration, or acid gas scrubbing (U.S. Pat. Nos. 4,347,711 and 5,600,968); waste water heat (U.S. Pat. No. 4,448,347); fumes from a heating boiler (U.S. Pat. No. 4,523,438); boiling solvent vapor (U.S. Pat. Nos. 4,537,660 and 4,539,816); waste heat heated water (U.S. Pat. No. 4,819,446); waste heat such as absorber heat, hot vapor heat, flue gases, or a combination thereof (U.S. Pat. No. 5,255,528).
A membrane separation method for removing water vapor from a gas is a method wherein a gas containing water vapor is contacted to one side of a vapor permselective membrane assembly, and a dry gas is contacted to the other side of the membrane, so that the water vapor is selectively permeated and separated through the membrane. In principle, it has merits over other three methods such that the running cost is low, the structure of the apparatus is simple, and dry air can continuously be obtained without polluting air. As a vapor permselective membrane excellent in permeability of water vapor, an ion exchange membrane as well as a dehumidifying method using such a membrane has been proposed by U.S. Pat. Nos. 3,735,558 and 4,909,810. Hollow fiber membrane-based dehydration is also known. See, for example, U.S. Pat. Nos. 4,783,201, 4,725,359, 4,718,921, 4,497,640, 4,583,996 and 3,511,031. U.S. Pat. No. 4,900,626 discloses a hollow composite fiber for dehydration having a polydimethylsiloxane coating on a dense layer of the fiber support.
Although membranes have been used in various separation applications, their use for heat pump systems has been limited. U.S. Pat. Nos. 4,152,901 and 5,873,260 propose to improve an absorption heat pump by using a semi-permeable membrane and a pervaporation membrane, respectively. U.S. Pat. No. 4,467,621 proposes to improve vacuum refrigeration by using a sintered metal porous membrane and U.S. Pat. No. 5,946,931 shows a cooling evaporative apparatus using a microporous PTFE membrane.